The results indicate that cannabis is not associated to higher health risks than alcohol, and that it has the same impact on relationships and professional life. However, the economic opportunities of individuals who are marijuana-dependant are reduced.

"Our research does not support arguments for or against cannabis legalisation, but it does show that cannabis was not safe for the long-term users tracked in our study," points out first author Magdalena Cerdá.

Downward social mobility

The publication is important, because it is one of the few which looks at cannabis' effects over such a long period of time, and not only for health, but for a wide range of socio-economic factors. The authors examined the medical data of 947 people part of the Dunedin Multidisciplinary Health and Development Study. All came from different social backgrounds.

The scientists assessed cannabis use and levels of dependence among the 947 participants. They looked at whether a regular use of the drug persisted at different phases of the study, when the participants came for their checkups, about every two years. Persistence of regular cannabis use was defined according to the number of study phases for which the participant reported using cannabis on four days a week or more.

Cannabis smokers encounter more money problemsGetty Images

The scientists also looked at professional, social and economic trajectories and examined whether the person had relationship problems. One of the most striking results was that, at age 38, those who had been considered dependant or regular smokers at various stage of the study, were significantly more at risk of getting less qualified, lower paid jobs than their parents. They were also more likely to suffer from money issues than people with an alcohol addiction.

"Our study found that regular cannabis users experienced downward social mobility and more financial problems, such as troubles with debt and cash flow, than those who did not report such persistent use," Cerdá says. "Regular long-term users also had more antisocial behaviors at work, such as stealing money or lying to get a job, and experienced more relationship problems, such as intimate partner violence and controlling abuse."

These socio-economic and relationship issues were still observed even after the scientists accounted for other factors such as problems during childhood, lower IQ, depression in adolescence, antisocial behaviours, abuse of other drugs and even criminal convictions.